Television transmission



Dc. 31, 1940. HANNS-HEINZ WOLFF ETAL 2,227,413

TELEVISION TRANSMISSION Filed Dec. 25, 1936 2 Sheets-Sheet l Dec. 31, 1940. HANNS-HEINZ WOLFF ETAL TELEVISION TRANSMISS ION Filed Dec. 23, 1936 2 Sheets-Sheet 2 Patented Dec. 31, 1940 UNITED STATES TELEVISION TRANSMISSION Hanns-Heinz Wolff and Gerhard Liebmann, Berlin, Germany; said Wolfi assignor, by mesne as signments, to Loewe Radio, Inc., a corporation of New York .Application December 23, 1936, Serial No. 117,362 In Germany December 18, 1935 1 Claim.

In television transmission devices, which employ image transmission tubes, the light-sensitive screen of which is constructed in the form of a storage electrode (mosaic photo-cells), it has been found that the contrasts of images with graduated light intensity diminish within fractions of seconds after the device being switched on, so that in place of a black-and-white image there is obtaineda dark grey-light grey image. It has been shown that the contrast cannot be adequately improved'by an increase in the amplification, as the danger is then encountered of certain interferenceswhich are likewise amplified at the same time, being increased accordingly. The reasons for this reduction in contrast have not been fully traced up to now. It is to be assumed, however, that there is developed in front of the mosaic screen a space charge cloud which cannot be removed sufficiently quickly.

If an attempt is made fundamentally to eliminate the stated diminishment in contrast, the danger exists of new errors being introduced. The present invention, however, sets forth a method of v eliminating or at least reducing the effect of this fault.

According to the invention, the amplifying device amplifying the potential impulses which are generated by the mosaic screen and which correspond to the light-intensity values of the individual picture elements is given a characteristic which has a certain special curvature.

By way of explanation of the invention, reference will be had to the accompanying drawings, of which Fig. 1 shows a device, of the type preferred in combinations according to the present invention, for producing vision signals, whereas Fig. 2 shows a characteristic curve of an amplifier as used according to the invention,

Figs. 3 and 4 show circuits which may be employed in carrying out the invention, and

Fig. 5 indicates how the circuit shown in Fig. l may be modified in certain cases.

Referring to Fig. 1, 3 is a photo-electric storage tube of the conventional type having a photoelectric storage structure in the form of a mosaic 4 backed by a signal plate l4. 5 is an optical device for projecting light from the phenomenon 6, an image of which is to be transmitted, onto the storage structure. An electron gun comprising a cathode 1, and exciting, concentrating and accelerating electrodes 8, 9, I and I I produces a scanning beam which is caused, by deflectors l2 and iii, to scan the storage structure for releasing therefrom electric impulses corresponding as to .shadow and brilliant ranges as compared with their chronological succession to the geometric distribution of brightness in the phenomenon 6. These impulses are coupled by way of a condenser l5, to the input of an amplifier of particular amplifying properties. This input is designated, in Fig. 1 as well as in Figs. 3 and 4, by the terminal Iii, to indicate that an amplifier circuit of particular properties, such as shown in Fig. 3 or 4, or'modifieol according to Fig. 5, is contemplated to be combined with a storage device, such as shown in Fig. 1, the combination ofsuch amplifier circuitswith storage devices being in fact of special importance (of. the introductory explanations).

In one preferred embodiment of the invention the characteristic curve relating input potential to output potential of the amplifier, is given the form of a reversed S, as shown in Fig. 2 of the drawings. This form of the characteristic causes within the range of small and large signalling potentials supplied by the mosaic plate, with small variations in the signalling potential, large variations in'the output potential, whilst in the medium range corresponding to signalling potentials of medium intensity it results, with small variations in the signalling intensity, also in small variations in the output potential. Owing to this form of the characteristic it is accordingly accomplished that the contrast is increased in the the range of medium light intensity. The operation may accordingly be referred to as a correction of a distortion of contrast. The electrical means for these corrections are known from the modern amplifying art, as the matter involved is merely the introduction of amplitude distortions.

It may also be desirable not to increase simultaneously the contrast of both the white portions and the shadows, but only that of one of these two ranges, for example the contrast in the shaded portions. This may be readily accomplished by corresponding modification of the characteristic of the amplifier.

Another means of obtaining a sharper contrast consists in providing the amplifier for the signals supplied by the storage photo-cell with a frequency characteristic in which the high frequencies are preferred. This also results in an increase in contrast insofaras sharp changes (black-and-white contours) are I accentuated, whilst comparatively large areas of an even tone are amplified to a smaller degree. This method accordingly has the disadvantage that the same, although providing an improvement in the 5 sharpness of the contours, nevertheless results in inferior reproduction of comparatively large areas of even tone. This method, which may be referred to as a distortion of the frequency, may also be readily performed on the basis of the modern amplifying art.

It is also possible to combine in suitable fashion the two methods above described for increasing the contrast of transmitted television images. In the arrangement according to Fig. 3, the image-content impulses are conducted for amplification purposes to the grids of two tubes I and 2 connected in parallel to each other and provided with grid biasses of such amounts that the one of the two tubes operates in the lower bend of its characteristic and the other in the upper bend of its characteristic. With suitable selection of the characteristics and the biasses it is possible so to superpose the two portions of the characteristics employed that a resulting characteristic is obtained having the form of a reversed S as shown in Fig. 2.

According to Fig. 4, for the purpose of obtaining an amplification which is greater in respect of the high frequencies than for the lower ones, the working resistance A or B in the anode circuit of the amplifying tube I and/or 2 may be constructed as a choke of suitable, and preferably high, inductance. There is then obtained an approximately linear frequency characteristic with an amplification factor increasing as frequencies increase. If it is desired to cause the amplification factor, dependent on the frequency, to increase to an augmenting degree as the fre quency increases, 1. e., to obtain a non-linear frequency characteristic having a curve which is concave in the upward direction, the chokes shown at the points A and/or B in Fig. 4 are preferably replaced by resonant circuits (Fig. 5), and care is taken that a suitable part of the resonance curve is operated on.

We claim:

In apparatus for transmitting visual phenomena wherein is included a photoelectric storage structure upon which is cast an image of a subject which is to be transmitted and wherein the said structure is scanned by an electron beam for releasing from said structure electric fluctuations in which the magnitude decreases from a substantially directly proportional value for illumination of the storage structure which approaches a maximum and a minimum intensity representing brightest and darkest picture areas and the magnitude of the fluctuation for intermediate intensities of illumination representing intermediate light values are substantially proportional to the illumination and wherein an amplifier is connected to amplify the said fluctuations, the method for compensating said distortions which includes the steps of increasing the amplification for both maximum and minimum intensity of said fluctuations as representing bright and dark picture areas, and maintaining substantially uniform and linear amplification for intermediate intensity fluctuations to represent intermediate light intensities.

HANNS-HEINZ WOLFF. GERHARD LIEBMANN. 

